Such an ultrasonic test using the reflection technique is known where the reflection of ultrasonic wave is received from a contact medium examining a target area of the test piece to be inspected. It is common that the reflection of ultrasonic wave is separated with time from the surface reflected waves.
When the ultrasonic test method for inspecting of disbonding is used for examining a layer assembly which has a CFRP (carbon fiber reinforced plastic) material bonded with an insulating layer, it execution is desired in a non-contact mode with no use of a contact medium. In such a mode as shown in FIG. 10, the ultrasonic wave is emitted from a probe 200 and their reflection B from a disbanding defect developed in the interface between the CFRP layer 101 and the insulating layer 102 is received which is however too low in the speed of propagation through the air to separate from but overlap with the surface reflected wave S reflected on the surface of the test piece 100 (the CFRP layer 101) as shown in FIG. 11.
Meanwhile, such a non-contact test method as disclosed in Non Patent Citation 1 using plate waves is proved that when the target area 103 to be inspected in a layer assembly is deep, its defect 103 may hardly be detected.
Non Patent Citation 1: “Ultrasonic inspection of CFRP material using air coupling”, by Shigeyuki Matsubara et al, the Institute of non-breakable testing, Japan, Proceeding of Autumn Meeting (Heisei 13), pp. 213-214.